The present invention relates to light directing films. In particular, the present invention relates to light directing films having a variable height structured surface.
Light directing films are used to increase the luminance of light exiting an optical display in a direction normal, or “on-axis,” to the surface of the display. One type of light directing film has a substantially planar surface and an opposing structured surface, which has an array of linear prism elements. Increasing the amount of on-axis light reduces the amount of energy required to generate a desired amount of on-axis luminance. This is particularly important for optical displays that use battery powered light sources such as those used in laptop computers, calculators, digital wristwatches, cellular phones, and personal digital assistants.
It is known in the art to place two light directing films closely adjacent to each other with the prism elements crossed at approximately 90° relative to each other in order to further increase the amount of on-axis light. However, an optical display using this structure may exhibit visibly apparent bright spots, streaks, or lines. This “wet-out” condition is caused by optical coupling between contacting, or very nearly contacting, surfaces of the adjacent films. To overcome this condition, the prism elements may be arranged in zones of taller and shorter prism elements to limit contact between the films. Representative embodiments of antiwet-out structures are described in U.S. Pat. No. 5,771,328, which is assigned to 3M Company.
The sides of each prism element intersect to form a peak or apex. The peak of the prism element is usually sharp, which is most effective at increasing the amount of on-axis light exiting the backlight. However, the sharp peak formed on each of the prism elements is relatively fragile making the film prone to scratching. This is particularly troublesome when the light directing films are being handled during the manufacture of devices such as those listed above, making it necessary to apply a pre-mask to the film after it is manufactured. The pre-mask protects the film from scratching, but must be subsequently removed prior to installation into an optical display. There is, therefore, a need for a light directing film that is scratch-resistant without substantially sacrificing the performance of the film.